The aerodynamics of a towplane in a kiting glider situation

On Monday, May 18, 2020 at 11:05:00 AM UTC-4, Tom BravoMike wrote:
On Monday, May 18, 2020 at 2:40:26 AM UTC-5, Marton KSz wrote:
I found this diagram last week:

https://photos.app.goo.gl/nLpWwLHFmgwE5dEM9

Here's my interpretation:

The system of the connected towplane + glider has a center of gravity, somewhere along the towrope, closer the towplane.

When the kiting begins, the system of the two connected masses start rotating around this CG (just like groundlooping a tailwheel aircraft).

However, since the glider is lighter, it rotates faster, which makes the impression that the towplane slingshots it.

Also, the glider has wings, and faster airspeed on the wings means more lift - the glider wants to go even higher.

All the energy for the extra lift + speed has to come from somewhe the supply is the kinetic energy of the towplane. As the kiting aggravates, the towplane drastically slows down. First it runs out of elevator control, then stalls.

That's quite an interesting picture, helping to understand the process and the discussion about the (initially) small angles.
The 800 ft minimum for the towplane to recover from a dive seems a lot. Could speak for using eg. touring motorgliders as tugs in hope they could recover faster.

In my low kiting incident I don't believe it started any more than 350 feet AGL. The 800 ft minimum appears to be if you are at 90 degrees nose down and stalled. I can't say exactly what angle I was at but It was beyond 60 I am sure, I recall no horizon in the wind screen. When the rope broke (which sounded like a mass of sheet metal hitting the ground) I realized I needed proper airspeed before I could fly and I didn't quite have it. I slowly eased out of the dive using what altitude I had and came in with power (which for some reason as I remember seemed a bit hesitant, I'm told maybe the G force on the float in the carb) and recovered slightly below the trees being visible in my peripheral vision off to the left. IF I was over the stand of trees, If it wasn't clear below me I would not have made it. At one point I thought I was going into the trees. That feeling sucks big time.

Walt Connelly
Former Tow PIlot
Now Happy Helicopter Pilot

The idea of deliberately diving the tug when this situation is
recognised is interesting - maintain energy, maintain elevator
authority, get rid of the glider, avoid the stall - sounds good. But I
guess the upset has already happened when the tug pilot first
recognises the situation. And who'd want to push into an even
steeper dive at 200'?


At 15:04 18 May 2020, Tom BravoMike wrote:
On Monday, May 18, 2020 at 2:40:26 AM UTC-5, Marton KSz
wrote:
I found this diagram last week:

https://photos.app.goo.gl/nLpWwLHFmgwE5dEM9

Here's my interpretation:

The system of the connected towplane + glider has a center of
gravity,
somewhere along the towrope, closer the towplane.

When the kiting begins, the system of the two connected masses
start
rotating around this CG (just like groundlooping a tailwheel
aircraft).

However, since the glider is lighter, it rotates faster, which makes
the
impression that the towplane slingshots it.

Also, the glider has wings, and faster airspeed on the wings
means more
lift - the glider wants to go even higher.

All the energy for the extra lift + speed has to come from
somewhe the
supply is the kinetic energy of the towplane. As the kiting
aggravates, the
towplane drastically slows down. First it runs out of elevator
control,
then stalls.

That's quite an interesting picture, helping to understand the
process and
the discussion about the (initially) small angles.
The 800 ft minimum for the towplane to recover from a dive seems
a lot.
Could speak for using eg. touring motorgliders as tugs in hope
they could
recover faster.

The thing that seems to be missing from these discussions (except for the tow from the CG) is the fact that the problem is the up force of the towline exceeds the ability of the tow plane elevator to resist. I do not know how much downforce the elevator can create, but it is a LOONG way from the breaking strength of the rope or even any weak link you can reasonably tow with. Near the breaking strength of the rope, I think you will find the angle that will provide enough lift on the tail to "win" over the elevator, will be quite shallow. And of course, that angle at the tail is the same change in angle that might happen from the tow pilot reacting to turbulence. All the thought of angle detection, make me think the inventors have never towed in rotor, where we see lock to lock control movement fairly regularly.

My favorites in this thread, are pushbutton release, no effort, fast, accessible, and yes, in bad cases, too slow. And perhaps the integrating load cell to look for a sustained high load, but we have no data to know if that is a valid condition to look for. A few of our upsets (thankfully at altitude) we from a glider release, in a climbing turn, that resulted in an instantaneous nose down of the tow plane, not the sustained load that is thought to happen in a kiting situation.

I am afraid, without a means to tow from a point on the tow craft that does not influence pitch, we are stuck with better, and recurring training.

RR

On Monday, May 18, 2020 at 12:40:26 AM UTC-7, Marton KSz wrote:
I found this diagram last week:

https://photos.app.goo.gl/nLpWwLHFmgwE5dEM9

Here's my interpretation:

The system of the connected towplane + glider has a center of gravity, somewhere along the towrope, closer the towplane.

When the kiting begins, the system of the two connected masses start rotating around this CG (just like groundlooping a tailwheel aircraft).

However, since the glider is lighter, it rotates faster, which makes the impression that the towplane slingshots it.

Also, the glider has wings, and faster airspeed on the wings means more lift - the glider wants to go even higher.

All the energy for the extra lift + speed has to come from somewhe the supply is the kinetic energy of the towplane. As the kiting aggravates, the towplane drastically slows down. First it runs out of elevator control, then stalls.

I fail to see the rational of designing a system that places a tow rope in front of the the tail feathers. Aren't you are just changing one failure mechanism for another?
Confused

On Mon, 18 May 2020 09:11:51 -0700, Jonathan St. Cloud wrote:

I fail to see the rational of designing a system that places a tow rope
in front of the the tail feathers. Aren't you are just changing one
failure mechanism for another?
Confused

At least some of the C47 glider tugs did just that.

An illustrated article: "GLIDER SNATCHING", THE AEROPLANE, JUNE 15, 1945
shows detail of the arrangement used to recover Hadrian troop gliders
from France, often with casualties on board.

The C47 had an electric winch with a variable braking system mounted at
the front of the troop/cargo area with the cab;e exiting from the
underside just behind the wing root fairings.

The glider being recovered had a short row rope ending on a large loop of
nylon rope supported on two light poles on front of the glider. The C47
did a low pass with a dangling hook, snagged the horizontal bit of the
rope between the poles and paid out line as the glider accelerated and
took off. Then the winch wound in the cable until the glider was at
normal towing distance and flew back to base towing the Hadrian.

https://www.youtube.com/watch?v=68O_ZHCOjwk

The next video shows both conventional glider towing, with the line
attached to the extreme rear of the C47 fuselage, and snatching:

https://www.youtube.com/watch?v=jfYSmGl8io8


....so anybody who'd been snatched low tow for the whole flight.




--
Martin | martin at
Gregorie | gregorie dot org

Winchlaunches for the entire USA.

I can't really contribute much to this discussion, as I am not a tow pilot and my engineering skills are more in line with less complex functions, but with current technology, target tracking (as previously suggested) has become more available and reliable. Perhaps a laser tracker at the tow hitch and a reflector at the glider end of the tow rope could be used to trigger either a guillotine or release actuator. Parameters could be set so that normal "boxing the wake" maneuvers would be permitted and ignored, but a high tow angle relative to the tow plane longitudinal axis would trigger the rope cut or release actuation. Of course, this implies a sudden and dramatic increase in the vertical positions of the ends of the tow rope prior to upsetting the tow plane axis. A gradual "kiting" motion may pull the tow plane tail up at a rate that the angle does not exceed the preselected deviance angle.

I also have to mention that getting any auto-release device approved by the FAA is the "elephant in the room" that nobody so far on this thread has considered. When Walt Connelly proposed banning Schweizer releases, I mentioned that there are some tow planes that are ONLY allowed to use Schweizer releases, and converting to a Tost system could (and would) require a lot of agonizing bureaucratic adventures with the FAA to get an STC or field approval to change the "approved" release to another "untested" installation.

I can only imagine the testing and documentation required to get an automatic release certified and approved. At best, the FAA is like a supertanker on course, and you are the guy in font of them in the rowboat furiously waving your handkerchief to make them turn. The Captain may see you and order an avoidance maneuver, but it's going to take a LOOOOONG time for the course to deviate one tiny little bit. Sort of like booting the rudder in a Grob 103.

Go ahead. Flame suit on.

On Monday, May 18, 2020 at 9:21:09 PM UTC-4, wrote:
I also have to mention that getting any auto-release device approved by the FAA is the "elephant in the room" that nobody so far on this thread has considered. When Walt Connelly proposed banning Schweizer releases, I mentioned that there are some tow planes that are ONLY allowed to use Schweizer releases, and converting to a Tost system could (and would) require a lot of agonizing bureaucratic adventures with the FAA to get an STC or field approval to change the "approved" release to another "untested" installation.

I can only imagine the testing and documentation required to get an automatic release certified and approved. At best, the FAA is like a supertanker on course, and you are the guy in font of them in the rowboat furiously waving your handkerchief to make them turn. The Captain may see you and order an avoidance maneuver, but it's going to take a LOOOOONG time for the course to deviate one tiny little bit. Sort of like booting the rudder in a Grob 103.

Go ahead. Flame suit on.

First of all, I am not a lawyer and I do not play one on TV. Just some interesting questions I have asked and as a result have become more confused, but that's lawyers for you.

Interesting that you mentioned dealing with the FAA over an "approved" release and in this case the Schweizer system. So, the FAA has approved this system and this system only for certain aircraft, I'm curious as to what aircraft those might be, not that this is really important. What is important is the concept under the law of "knew or should have known." The FAA had to know (unless the FAA has been living under a rock) that the "approved" system has failed miserably just when it is needed the most resulting in the death of tow pilots in the past.
And what "untested' installation do you mean? The TOST system has been tested thoroughly and although any system can fail the consensus is that the TOST system is superior to the Schweizer. Prove me wrong.

The "Federal Tort Claims Act" allows for legal action being taken against Federal agencies in the event of situations such as this, knew or known or negligence. The old rule that you can't sue the Sovereign (government) no longer applies in all cases. Plenty of suits against VA doctors and other successful legal actions I am told. While these are rare events if a family with standing gets the right legal team going after the major league DEEP POCKET, (the GUBMENT) then things might change. Then again, maybe not. Lawyers only go after cases where they believe they can win and secure a large judgement, a nice percentage of which goes in their pocket.

Additionally I was previously told that Soaring is a sport and that the tow pilot is essentially engaging in the sport and therefore assumes a degree of risk. Perhaps, perhaps not I am informed by another "legal mind" who actually is a pilot. The sport arguably doesn't begin until the gate opens for the "race." Therefore it might be argued that the tow pilot is not a competitor and therefore not engaged in the sport in either towing for a competition or simply launching a glider. (ask four lawyers a question and get 6 different answers) If in the case of the tow pilot in California the family has "standing" meaning that they depend on him for their livelihood they might well have a reasonable chance of prevailing in a law suit. (let that be a lesson to you commercial operators and clubs, only have tow pilots without wives, children, families depending on them or you could lose you ass) The glider pilot in this case was an instructor as we have been informed, someone who "Knew or Should Have Known" that his actions might imperil the tow pilot. In this case the pilot failed to take proper care in doing something resulting in death. No one believes for a moment this was purposeful on his part, but the right lawyer can sue the sun for going down and the tide for going out and prevail.

The real problem is the safety culture of the community as a whole. Until commercial operators and clubs take it upon themselves to recognize failed systems and correct them these things will continue to happen. You can talk about training until hell freezes over but when a human makes an error the tow pilot still has every right to the best reasonable escape mechanism, not one that is proven to have failed when needed the most. People still still make mistakes, nothing will ever make flying or towing completely safe but actions, not words are what makes thing safER. Take action my friends, the life you save may be the tow pilot.

Walt Connelly
Former Tow Pilot
Now Happy Helicopter Pilot.

Just because one system is superior and safer to another has nothing to do with whether it is approved for installation by the FAA. The documentation for installing a tow release on a particular aircraft is clearly spelled out. If it permits a Schweizer hook, then that's what has to be installed to pass inspection. Changing to a Tost is not in compliance with the documentation, so it would not pass inspection.

Getting a Tost approved for retrofit to an aircraft for which it is not specifically approved can be an arduous and time consuming process. ANYTHING having to do with the regulatory environment at the FAA is a long and painful experience. Bob Carlton and I spent NINE YEARS getting the FAA to agree to a method to extend the life limit on the Pegasus. And that was an instance where the FAA had obviously made a mistake, violated their own procedures and the Administrative Procedures Act and still "circled the wagons' to defend the one guy who pushed the life limit AD through.

I am certainly not arguing that getting a Tost approved for all aircraft would be a desirable thing. It is just that EACH aircraft type would have to be individually approved. There are very few if any "blanket" approvals across aircraft types for mechanical devices. Avionics are another story, but we aren't talking about that type of equipment.

As far as releasing liability, the big question is whether the tow pilot or his employer (club or commercial) is providing an aviation service for compensation. If the glider pilot is paying for the tow, then it is fairly obvious. However, as you say, four lawyers can easily deliver six opinions.